1. Field of the Invention
The invention relates to an image processing system for binarizing an image signal obtained by reading an image of an original.
2. Related Background Art
Hitherto, as an image reading apparatus, there is an apparatus such that an original reading function is added to a word processor. For example, a scanner unit having therein a line image sensor is installed to a carriage for a serial thermal transfer printer and by setting a reading original in place of a printer recording paper, the original is read. In such an apparatus, an image existing in an area of (reading width of the line image sensor).times.(one scanning width of the carriage) is read by one scan of the carriage, the reading original is subsequently fed in the direction which perpendicularly crosses the carriage scanning direction by a distance of the reading width of the line image sensor, and after that, the carriage is scanned once, thereby reading an image of the same area as mentioned above. By repeating the above operations, an original image of a desired area can be read.
There is also a multifunction printer such that a scanner cartridge of the same shape is installed in place of an ink cartridge on a carriage of an ink jet printer and an original image can be read by an operation similar to that of the serial thermal transfer printer.
According to the serial scanner system as mentioned above, since the number of reading elements of the line image sensor is smaller than that of the line scanner system using the line image sensor having pixels of the number corresponding to the original width like a flat bed scanner, a small sensor or optical system can be used. Since an original paper feeding system and the carriage can commonly use a mechanism of the printer as it is, there is no need to buy both of the printer and the scanner. As mentioned above, the user can fairly cheaply get the original reading function. A hand-written illustration or a photograph which was read by the reading function can be inserted into an original document or an original can be read for an OCR or a facsimile apparatus.
When an original to be read is not a photograph having gradations but a monochromatic characters or illustration, multivalue data from the line image sensor is ordinarily binarized by a predetermined threshold value. In this case, there is a problem such that a background (white background of a paper in case of an ordinary character original) of the original to be read is not constant. As an original, there is a high quality paper, a recycling paper, a rough paper, a newspaper, a colored paper, or the like. Even when characters or lines of the same black are drawn, when they are binarized by the same threshold value, there is a case where the line is made faint and patchy or is thickened, so that the characters or the like cannot be correctly read.
A conventional binarization reading apparatus such as a facsimile or the like has a background control function. FIG. 2 shows a background control of the facsimile or the like. Multivalue data before a binarizing process from the left side is first inputted to a terminal B of a comparator 201. A count value of a counter 202, which will be explained hereinlater, is inputted to a terminal A of the comparator 201. FIG. 2 shows an example in which as a color becomes white, a data value increases. As a result of a comparison, when A&gt;B, "1" is subtracted from the count value of the counter 202 and, when A&lt;B, "1" is added to the count value. Such a process is not performed every pixel but is executed while being thinned out by thinning-out units 203 and 204. This is because it is necessary to prevent a situation such that a binarization threshold value, which will be explained hereinlater, excessively changes and exerts an adverse influence on the binarization result. Since most of the area of the ordinary original to be read corresponds to a background, the value of the counter 202 is converged to a value of the background. Such a value is divided into 1/2 by a divider 205, thereby setting to a binary threshold value. The multivalue data before the binarizing process is binarized by a comparator 206.
In a line scanner like a flat bed scanner, even if the background changes even in one original in the foregoing background control system, the threshold value smoothly changes every line scan of the line sensor, so that a good binarization result is obtained.
According to the serial scanner as mentioned above, however, although the threshold value smoothly changes in the scanning direction of the carriage (subscanning direction of the line sensor), a large difference occurs in the threshold values between adjacent scanning areas, so that there is a case where an unnaturality occurs in the binarization result. For example, as a result of that a line on the original is read, if the line exists in the adjacent scanning areas, there is an unnaturality such that the line width suddenly changes or the like.